Fall is the best time of year for controlling many perennial weeds, including Canada thistle, leafy spurge, quackgrass, and common milkweed, since the plants are preparing for winter by storing nutrients in their root systems. This improves herbicide translocation to the roots, and if you kill the roots, you kill the plant.

Some factors to consider with fall perennial weed control, from the Extension Service at North Dakota State University and the University of Minnesota:

• With most perennial weeds, treatment is generally better when the plants are larger, with full leaves or bloom, since more of the herbicide will translocate down to the roots.

• Banvel, Roundup, Tordon, 2,4-D, Curtail, and Stinger are herbicides that have the greatest activity on Canada thistle. Patch spray common milkweed with Roundup Ultra and ammonium sulfate.

• Highest rates should be used without interfering with next year's cropping pattern.

• Situations may occur where spot treatment with herbicides at high rates might be the better perennial weed management option, even if carryover may injure small areas of next year's crop. Small patches are easier and less expensive to treat than entire fields. But bear in mind, research has shown at least 90% stand reduction from Banvel, Roundup, Landmaster BW, or Tordon plus 2,4-D.

• Apply herbicides when temperatures are expected to exceed 60-65 F during the day to ensure active translocation.

• Timing is critical for fall control. Herbicides should not be applied to perennial weeds that are stressed from drought or frost damage, although light frost in the fall can actually enhance perennial weed control efforts. Rather than killing the perennial weed, the frost signals it to store nutrient reserves in its roots. Thus, you can treat perennial weeds with a systemic herbicide, as long as the plants are still growing and have enough healthy vegetative leaf matter to absorb the herbicide.

• It usually takes a hard frost to freeze and wilt the leaves of perennial weeds, effectively ending the opportunity for chemical control until the next growing season. When frost temperatures below 26-28 degrees occur, wait at least 24 hours to evaluate foliage before spraying. Herbicides cannot translocate if the weed is dead.

• Select the correct herbicide that fits the crop rotation sequence. Use the highest rate labeled. Some herbicides have crop rotation restrictions, so pay attention to the label.

• Perennial weed control will be greatest if the field has not been tilled prior to the herbicide application. However, control is enhanced by tillage after a herbicide application. – Tracy Sayler